As others have suggested, circumstantial evidence is simply evidence that tends to prove an issue by proving other events (circumstances) from which the occurrence of the matter at hand can be reasonably inferred.
It’s generally used in contrast to direct evidence, which establishes the factual matter to be proved by it without the need for inferences.
If you step into a room just in time to see me pulling a knife from the bloody chest of my sworn enemy, and plunging it back into his now-lifeless body, you have only circumstantial evidence that I killed him. After all, you didn’t see me stab him. The only direct evidence you can provide is that I was merely removing the knife. The second stab wound was inflicted on an already lifeless body; at best, I have mutilated a corpse, not killed a man.
But it’s a very reasonable to infer that I stabbed him. The fact-finder (judge or jury) is permitted to make reasonable, common-sense inferences from the evidence that is presented, and decide what weight to give to each.
When people say, “That’s only circumstantial evidence,” perhaps they mean, “The inference you are drawing from these circumstances is not a strong one.” 
- Rick